1. Field of the Invention
The present invention generally relates to an optical transmitter, and more particularly to optical transmission method and apparatus for generating optical duo binary signals with frequency chirp.
2. Description of the Prior Art
In the information age, the demand for optical networks of higher data capacities is constantly increasing. This demand is fueled by many different factors, such as the tremendous growth of the Internet and the World Wide Web. Optical fiber transmission has played a key role in increasing the bandwidth of telecommunications networks. It is the preferred medium for transmission of data at high data rates and over long distances nowadays.
At very high data rates, an inherent chromatic dispersion property, which means different spectral components of the signal traveling at different speeds, in optical fiber transmission fibers causes waveform deterioration and becomes a limiting factor in standard single-mode fiber (SMF). Since there is a large installed base of SMF, a great demand for dispersion tolerant data transmission systems exists.
In standard optical communication systems, such as SONET, PDH, and SDH, data rates are in a hierarchy of 155 Mbps, 622 Mbps, 2.5 Gbps, and 10 Gbps with a multiply factor of four. In SDH terminologies, the data rates above are referred as STM-1, STM-4, STM-16, and STM-64. Chromatic dispersion becomes important when data rate is counted in Gbps magnitude. Here, 10 Gbps data rate is taken as an example. In this regards, the chromatic dispersion of a standard SMF is at 17 ps/nm*km at 1550 nm. The spectral width of a chirp-free optical signal is given by the Fourier transform limit, i.e., the width is equal approximately to the inverse of the minimum pulse duration, or the data rate. Thus for a NRZ (non-return-to-zero) binary signal at 10 Gbps, the minimum pulse duration is 100 ps and the spectral width is around 10 GHz or 0.08 nm. After 70 km transmission in SMF, the signal pulses would be broadened by around 100 ps, the minimum pulse duration or the bit period. Therefore the dispersion limited transmission distance of a chirp-free 10 Gbps NRZ optical signal is about 70 km in SMF.
Various methods, such as ODB (Optical Duo-Binary) modulation, were invented to extend the dispersion limited distance. The characteristic of an ODB signal is that it has three phased modulated states, −1, 0, and +1 while it maintains two states of intensity, which follows the input NRZ signal levels. There is no direct transition between −1 and +1 state. Due to its property, ODB signal has narrower spectral width than NRZ signal. Therefore, ODB signal could be transmitted farther in distance than NRZ signal in SMF.
A classical method of generating ODB signal was described in U.S. Pat. No. 5,543,952, “Optical. Transmission System” by Yonenaga, et al. A three-level electrical signal is generated by combining an input binary signal with its exact 1-bit delayed replica. Then the three-level electrical signal is used to drive a MZ (Mach Zehnder) interferometer type modulator biased at null to generate the ODB signal. At 10 Gbps data rate, dispersion limited transmission distance of 120 km in SMF is obtained by applying the ODB signal. In addition, sharp cut-off filtering of the driving signal is proposed by K. Yonenaga and S. Kuwano in “Dispersion-Tolerant Optical Transmission System Using Duo-binary Transmitter and Binary Receiver”, J. Lightwave Technol, Vol. 15, pp. 1530-1537 (1997).
A partial response method of generating ODB signal was described in U.S. Pat. No. 5,867,534, “Optical Transmission Method with Reduced Sensitivity to Dispersion, Transmission Device and System For Implementing this Method” by Price, et al. A quasi three level electrical signal is first generated by passing an input NRZ signal through a narrow filter with bandwidth about 25%-30% of the data rate. Then the three-level electrical signal is used to drive a MZ modulator biased at null to generate the ODB signal. At 10 Gbps data rate, dispersion limited transmission distance of 210 km in SMF is obtained by applying the partial response method.
It is recognized that the longer transmission distance of an ODB signal is not entirely due to the narrower bandwidth. The “bumps” in the optical signal near the “0” bit state also plays a very important role in extending the transmission distance. The “bumps” are by-products of both partial response method and classical 1-bit delayed method using sharp cut-off filters. Since the bumps have 180 degrees phase shift to the adjacent “1” bit, the destructive interference between the bump and the adjacent “1” bit would help to confine the broadening of the “1” bit pulse. Hence the dispersion penalty would be reduced accordingly. Due to its long transmission distance and its simple implementation, the partial response method is the most widely used ODB method, and is often referred as THE ODB method.
The “bumps” can be also introduced using a fractional delay, instead of the whole one-bit delay, in the classical ODB method. This method is described in U.S. Pat. No. 6,623,188, “Dispersion Tolerant Optical Data Transmitter” by Dimmick, et al. A four-level electrical signal is generated by combining an input binary signal with its delayed complement through a differential amplifier in this disclosure. The four-level electrical signal is then used to drive a MZ modulator biased at null to generate a four-level optical field, which could be transmitted of 150 km in SMF without any band-limiting filters at 10 Gbps data rate.
Theoretically, combining an input binary signal with its delayed complement through a differential amplifier is equivalent to combining the input binary signal with its delayed replica. Within the frequency domain, this is equivalent to passing the signal through a periodical filter with a frequency response, formulated as:
                              Filter          ⁢                                          ⁢                      (            f            )                          =                              1            +                          ⅇ                                                ⅈ                  ·                  2                  ·                  π                  ·                  f                  ·                  Δ                                ⁢                                                                  ⁢                t                                              2                                    (        1        )            where Δt is the time delay. In contrast, the partial response method requires low pass filters with smooth falling “tails” at high frequencies. The combined effect of the Equation 1 and the intrinsic bandwidth of the driving circuit lead to the fractional delay method giving similar results as the partial response method.
Using frequency chirp for extending transmission distance was described in U.S. Pat. No. 6,337,756, “Optical Transmitter System and Method”. A method to generate a classical 1-bit delayed ODB signal with frequency chirp was disclosed. Simulation shows that with small negative frequency chirp, transmission distance could be slightly improved over classical ODB.
Chirped ODB modulation was studied in “Chirped duo-binary transmission for mitigating the self-modulation limiting effect”, presented at the OFC 2001, March 2000 by M. Wicher, et al. It shows that negative chirp could improve signal quality at shorter distance (<150 km at 10 Gbps) while positive chirp could improve signal quality at distances greater than 150 km, for example, a positively chirped signal was found to transmit greater than 250 km. In addition, the positive chirp vas also found to reduce self phase modulation effect.
In the description of “Cost-effective optical chirped duo binary transmitter using an electro-absorption modulated laser”, IEEE Photon Technology Letters, Vol. 17, pp. 905-907, April 2005 by Hanlim Lee, et al; positively chirped ODB signals were generated with a combination of electro-absorption modulated laser (EML) and a MZ modulator. A similar result was obtained by demonstrating that positively chirped ODB signal at 10 Gbps could be transmitted over 250 km in SMF at wavelength around 1550 nm.
FIG. 1 is a schematic diagram illustrating a conventional ODB transmitter 100. An electrical. NRZ source 110 is fed into a pre-coder 120, which is used to make the final optical output signal intensity of this transmitter 100 to be as the same as the input NRZ source 110. In one case, the pre-coder 120 could be a simple XOR gate, with one input being connected to the complement of the incoming NRZ data signal and the other input being connected to the one-bit delayed XOR gate output. The output of the pre-coder 120 is sent into an ODB encoder 130, which produces a three-level or a four level signal ce(t) to a data driver 140. As described earlier, the ODB encoder 130 could be implemented using the delay-and-combine methods or using partial response method. The output of the ODB encoder 130 is amplified by the data driver and forwarded to drive a MZ modulator 150, which also receives optical source from a laser apparatus 160. The optical output of the MZ modulator is an ODB signal where the optical intensity I(t) follows the input NRZ signal d(t).
The partial response method is the simplest one to generate chirp-free ODB signals with large dispersion tolerance. However, all chirp-free ODB signals are very sensitive to non-linear distortion. For example, SPM (self phase modulation) on the transmitting signals can significantly reduce their transmission distance because SPM makes a negative frequency shift at the rising edge and a positive frequency shift at the falling edge, which is generally referred to as a negative chirp. Coupled to the dispersion, the frequency domain distortion is converted to time domain distortion, which is the source of non-linear penalty.
Therefore, a positive pre-chirp could be intentionally added to the transmitting signal to compensate the SPM caused negative chirp in order to reduce introduced non-linear penalty. The chirped ODB methods should have better performance with higher signal launch power. However, the methods disclosed in prior art used double optical modulations, which require precise time alignment between the electrical signals driving the two modulators. These two driving signals in the chirped ODB methods are data patterns; one is NRZ signal and another is pre-coded data. The variable delay lines needed for the timing alignment have to have uniform response over a broad bandwidth, which are usually bulky and expensive. This is a big obstacle for the practical implementation of the chirped ODB modulation methods.
Thus there is a need for an improved and practical method to generate chirped ODB signals.